The invention is based on an electrical incandescent lamp, in particular a halogen incandescent lamp, having an IR-reflective coating, as claimed in the preamble of claim 1.
In particular, the invention relates to an incandescent lamp having a flat luminous body, for example a so-called flat-core filament. Unlike the incandescent filaments in incandescent lamps for general illumination, the cross section of flat-core filaments does not have a circular cross section but, in fact, an elongated cross section. The reason for this is to match the geometry of the filament shape to that emission characteristic of the lamp or of the incandescent filament that is preferred for the respective main application. In contrast to the rotationally symmetrical emission characteristic of conventional incandescent lamps, the flat luminous body of the lamp types mentioned initially allows particularly flat emission as is desirable, inter alia, for technical/scientific illumination purposes and in photographic optics, in particular for projection purposes. Typical electrical power levels are in the range from about 50 to 400 watts.
The coating which reflects IR radiation and is applied to the inner and/or outer surface of the lamp bulbxe2x80x94referred to for short as the IR coating in the following textxe2x80x94results in a large proportion of the IR radiation power emitted from the luminous body being reflected back to it. The improvement in the lamp efficiency achieved in this way can be used firstly to increase the temperature of the luminous body, and in consequence to increase the light flux, with a constant electrical power consumption. Secondly, a predetermined light flux can be achieved with a reduced electrical power consumptionxe2x80x94an advantageous xe2x80x9cpower saving effectxe2x80x9d. A further desirable effect is that the IR coating results in considerably less IR radiation power being emitted through the lamp bulb, and thus in the environment, for example an optical projection apparatus, being heated less than by conventional incandescent lamps.
Owing to the unavoidable absorption losses in the IR coating, the power density of the IR radiation elements within the lamp bulb decreases with the number of reflections and, in consequence, the efficiency of the incandescent lamp also decreases. The critical factor for the efficiency increase which can actually be achieved is therefore to minimize the number of reflections required to return the individual IR beams to the luminous body. To this end, the shape of the lamp bulb that is provided with the IR coating is specifically matched to the shape of the luminous body.
EP-A 0 470 496 discloses a lamp having a spherical bulb, in whose center a cylindrical luminous body is arranged. This document teaches that the reductions in efficiency due to the difference between the luminous body and the ideal spherical shape can be limited to an acceptable level, subject to the following preconditions. Either the bulb diameter and the luminous body diameter or length must be carefully matched to one another within a tolerance band, or else the diameter of the luminous body must be considerably less (the factor by which it is less being 0.05) than that of the lamp bulb. Furthermore, a lamp having a rotationally ellipsoid bulb is cited in whose focal line an elongated luminous body is arranged axially.
The invention is based on the object of specifying an incandescent lamp having a flat luminous body, which incandescent lamp is distinguished by the emitted IR radiation being returned efficiently to the luminous body, and in consequence being distinguished by high efficiency. A further aspect is the distribution of the return IR radiation on the luminous body. Furthermore, the aim is to achieve compact lamp dimensions with high light densities, as is particularly desirable for low-voltage halogen incandescent lamps.
This object is achieved according to the invention by the descriptive features of claim 1. Particularly advantageous refinements can be found in the dependent claims.
The invention proposes that the lamp bulb be deliberately shaped such that the lamp bulb has no rotational symmetry with respect to the axes lying in the plane of the light of the flat luminous body, but the lamp bulb in fact has a shape which differs from rotational symmetry but is matched to the flat geometry of the luminous body, that is to say a flattened shape. If required, the shape of the base area of the flat luminous body can also be matched to the area of the luminous body that is actually illuminated by the reflected IR radiation.
In particular, the invention proposes that the shape of the lamp bulb corresponds essentially to an ellipsoid having three half-axes, at least two of which are of different lengths, and with the luminous body being arranged inside the lamp bulb in such a manner that the shortest of the three half-axes of this ellipsoid is oriented at right-angles to the plane of the light of the luminous body. In this way, the lamp bulb has the desired flat shape, when viewed in the direction of the plane of the light.
The following text refers to FIGS. 1a-1c in order to explain the idea of the invention further. These Figures show a schematic illustration of the fundamental relationships, and introduce a number of parameters which are essential for further understanding of the invention.
The Figures show, inter alia, three sections through an ellipsoid 1 having the three half-axes a, b and c. The sections correspond to the viewing directions in the three Cartesian spatial axes x, y and z, which are also chosen to be collinear with the three half-axes a, b and c, respectively. The half-axis c is shorter than the other two half-axes a and b. A stylized flat luminous body 2 having two mutually parallel rectangular base areas 3 and 3xe2x80x2 is arranged centered inside the ellipsoid 1. These two base areas 3 and 3xe2x80x2 correspond in the case of a real flat luminous body to those two luminous areas which essentially produce the light flux of the lamp.
For simplicity, the following text refers to a fictional plane of the light, which is defined as running parallel and centrally between the two base areas 3, 3xe2x80x2.
The luminous body 2 is now oriented inside the ellipsoid 1 in such a manner that its fictional plane of the light is at right-angles to the half-axis c. The half-axes a and b thus run in the plane of the light and, in consequence, parallel to the base areas 3, 3xe2x80x2 of the luminous body 2.
The specific values for the three half-axes can be deliberately individually matched to the shape and dimensions of the luminous body in such a way as to ensure that the emitted IR radiation is returned as efficiently as possible to the luminous body. In order to achieve a long lamp life, the weighting of the matching criteria for the three half-axes may also be shifted more in the direction of the returned IR radiation being distributed as uniformly as possible on the luminous body. In particular, local IR radiation power maxima, so-called xe2x80x9chot spotsxe2x80x9d, generally have a detrimental effect on a long life of the luminous body, and should therefore be avoided.
The uniformity of the distribution can also be improved by deliberately matching the external shape of the luminous body to the shape of the returned radiation spot on the luminous body. For example, it has been found that the returned radiation spot is essentially oval when the maximum amount of emitted IR radiation is returned. For this reason, it may be advantageous to chose an oval shape for the luminous body as well and, furthermore, largely to match its external dimensions to those of the returned radiation spot.
In the case of rotationally symmetrical luminous bodies, that is to say luminous bodies having a circular base area, and in the case of luminous bodies which may be regarded, at least to a rough approximation, as being circular for example luminous bodies having a square base area, it may be advantageous to chose the half-axes a and b of the ellipsoid to be of equal lengths.
The deliberate matching of the three ellipse half-axes to the luminous body can be assisted using so-called ray-tracing methods. In this case, light beams that originate from the flat-core filament are traced, and the ellipse half-axes are determined, in such a way as to achieve maximum efficiency for returned radiation, or else optimum uniformity of the distribution of the returned light beams on the filament, or some compromise of this nature.